This invention relates to a high voltage d.c. power transmission system that utilizes a metallic conductor as a return, or neutral, conductor for carrying return current between opposite ends of the system. More particularly, the invention relates to means for protecting the return conductor and associated equipment against overvoltages produced by system transients.
An HVDC system is sometimes called upon to operate in a monopolar metallic return mode. This is a monopolar mode in which the d.c. current returns through a separate metal conductor instead of through the earth or ground, sometimes referred to hereinafter as earth ground. In such a system, one end of the return conductor is grounded while the other end floats with respect to d.c. The floating end is often very remote from the grounded end. For example, in one HVDC system presently in operation, i.e., the Square Butte system operating between Center, N.D. and Arrowhead, Minn., the distance between the two ends is 465 miles.
Under steady-state conditions, the neutral voltage at the floating end of the return conductor is equal to the d.c. "IR" drop of the return conductor. In the Square Butte system referred to above, this steady-state voltage is about -16 kV, assuming converter 12 of FIG. 1 has the polarity depicted therein.
The voltage level of the neutral under steady-state conditions pesents few insulation problems. But large overvoltages can occur on the return conductor during transients, such as converter bypass, commutation failures, starts, restarts, a.c. voltage transients, and d.c. line faults. All these conditions cause overvoltages to be superimposed on the steady-state voltage.
Such overvoltages require either that the insulation level of the neutral, at its floating end, be very high or that some means be provided to hold down the level to which the overvoltage rises. In a technical paper appearing in the IEEE Transactions on Power Apparatus and Systems, 1971, pages 554-563, N. R. Hingorani describes an arrangement for implementing the latter approach.
More specifically, Hingorani connects between the neutral conductor and ground, at the floating end of the neutral conductor, the parallel combination of a large capacitor and a gap-type lightning arrestor. There are several problems associated with this type of apparatus. First, the capacitor must be large enough so that the gap-type arrestor does not spark-over for most operating transients, and such a capacitor is quite expensive. In one existing system, this capacitor has a value of 50 .mu.f. A second problem with this prior apparatus is that harmonics generated by the converters tend to pass through the large capacitor and ground in preference to the metallic return conductor inasmuch as the capacitor and ground have a lower harmonic impedance than the metallic return. The resulting harmonic current through ground is a major cause of telephone interference.